Hitherto, for a switching power supply circuit, a separately-excited switching power supply system in which switching is made by an output of an OSC (Oscillator) and a self-excited switching power supply system which does not use the OSC are used. According to the self-excited switching power supply system, although it can be constructed at a lower cost than the separately-excited switching power supply system, when a load decreases, an oscillation phenomenon occurs inevitably.
In the self-excited switching power supply system, therefore, in order to suppress the oscillation phenomenon, the switching power supply circuit is designed to have only low efficiency. In other words, it cannot be designed so that high efficiency is obtained at the maximum load.
It is, therefore, an object of the invention to provide a switching power supply apparatus and a power control method in which even in the self-excited switching power supply system, a switching power supply circuit is designed so that the high efficiency operation is executed at the maximum load and, even if a capacity of a load fluctuates, a voltage and a current can be stably outputted.